The neonatal Fc receptor (FcRn) is a heterodimeric protein consisting of a transmembrane MHC class I-like heavy chain (FcRn α-chain) and the β2-microglobulin light chain, the latter also forming a part of MHC class I molecules (Simister and Mostov (1989) Nature 337:184-7; Burmeister et al. (1994) Nature 372:379-83).
FcRn is predominantly located in endosomes and is able to bind to serum albumin and immunoglobulin G (IgG) at pH≤6.5 and release them at pH≥7.0 (reviewed in Roopenian (2007) Nat Rev Immunol 7:715-25).
FcRn carries out several distinct tasks in mammals (reviewed in Roopenian, supra). FcRn is involved in recycling of endocytosed IgG and serum albumin, thus avoiding their degradation in the lysosome, giving them longer half-life and higher availability in the blood than other serum proteins. When IgG, serum albumin and other serum proteins are passively pinocytosed by cells in contact with blood, the pH becomes gradually lower in the formed endosomes, which permits the binding of IgG and serum albumin to FcRn. The receptor is then, together with its bound ligand, transported via recycling endosomes back to the plasma membrane. After returning to the plasma membrane, the pH increases to above 7, at which point the bound ligand is released.
FcRn is also recognized for its ability to transport IgG over barriers such as the placenta, the upper airway epithelium, the blood-brain barrier and the proximal small intestine.
In mammals, the properties of FcRn are used to transcytose IgG from a mother to a fetus via the placenta, and to transcytose IgG from a mother's milk to the blood stream of an infant in the proximal small intestine.
The expression pattern of FcRn differs between species. However, FcRn is widely expressed by cells in the blood brain barrier, upper airway epithelium, kidneys and vascular endothelia, and by antigen presenting cells as well as by other cells of hematopoietic origin in most species (reviewed in Roopenian (2007), supra).
Antibodies and peptides with affinity towards FcRn (Liu et al. (2007) J Immunol 179:2999-3011, Mezo et al. (2008) Proc Natl Acad Sci USA 105:2337-42) and β2-microglobulin (Getman and Balthasar (2005) J Pharm Sci 94:718-29) have been developed with a view to inhibit the binding between endogenous IgG and FcRn. Another approach has been to mutate the Fc region of the IgG to get a higher affinity for FcRn (Petkova et al. (2006) Int Immunol 18:1759-69, Vaccaro et al. (2005) Nat Biotechnol 23:1283-8).
Fusion to the Fc domain or to albumin is a widely used strategy to increase the in vivo half-life of proteins. However, the large size of such fusion proteins adversely affects tissue penetration and reduces the specificity to the fusion partner (Valles et al. (2011) J Interferon Cytokine Res 32:178-184). On the other hand, mutations have been made in the Fc fragment of antibodies administered to non human primates to prolong half-life (Hinton et al. (2004) J Biol Chem 279:6213-6). However, this approach is only limited in use to therapeutic antibodies, and cannot be extrapolated to other therapeutic proteins unless the proteins in question are fused to Fc fragments, which also results in large size molecules. A number of chemical and recombinant methods have been devised to improve protein half-life, such as PEGylation and genetic fusions of the protein to the Fc domain of IgG or albumin (reviewed in Schellenberger et al. (2009) Nat Biotechnol 21:1186-1190). PEGylation of proteins has been reported to decrease their potency and contribute to their immunoreactivity.
Fc-fusion proteins have also been used for oral and pulmonary delivery mediated by the FcRn (Low et al., (2005) Human reproduction July; 20(7):1805-13), however similar problems relating to tissue penetration and reduced specificity remain, due to the size of the fusion molecules.
Hence, there is large need in the field for the continued provision of molecules with high affinity for FcRn. In particular, small binding molecules are needed that, when present as a fusion partner, do not adversely affect the properties of the molecules they are fused to and do not contribute to immunoreactivity.